Microenvironmental Modulation of Bromocresol Green upon γ-Cyclodextrin Association Characterized by UV–Vis Spectral Trajectory Analysis

Cyclodextrins (CyDs) provide a microenvironment distinct from the bulk solvent by incorporating guest molecules within their hydrophobic cavities; thus, they can induce changes in the physicochemical properties of drugs. In this study, we investigated how γ-CyD association alters the microenvironment experienced by a guest molecule, taking bromocresol green (BCG) as a model reporter molecule. BCG was selected because its ultraviolet–visible (UV–Vis) spectrum sensitively reflects acid–base equilibrium and local solvation-related changes. The results indicated that, upon γ-CyD addition, the spectral contribution assigned to the dianionic form of BCG increased. In addition, a bathochromic shift was observed for the absorption band assigned to the dianionic form. By applying singular value decomposition (SVD) to the UV–Vis spectra, spectral variations associated with acid–base equilibrium and bathochromic shifts were empirically resolved as dominant components. Through trajectory analysis of these components, the γ-CyD-induced microenvironmental effect on BCG was expressed in terms of reference scales based on pH-dependent and dimethyl sulfoxide (DMSO)/water-dependent spectral changes. Based on this trajectory analysis, the γ-CyD-induced spectral changes of BCG were operationally expressed as corresponding to an apparent acid–base equilibrium state comparable to that of an aqueous solution at pH 5.66 and to a bathochromic response observed in a 50.8% DMSO/water mixed solvent. These results indicate that γ-CyD association alters the apparent microenvironment experienced by BCG and that SVD-based trajectory analysis enables this effect to be described in terms of physically interpretable spectral reference coordinates.